This study will evaluate the role of bronchial reactivity as measured by methacholine responses and by variability in peak flow in the etiology and natural history of airway obstructive diseases (AOD). It will evaluate the interactions of bronchial reactivity, host immunological status, smoking, environmental and occupational exposures in these processes and in the differentiation of different types of AOD. It will study further the development and course of what appear to be (from our prior studies) the two major types of AOD, from both the antecedent conditions and patterns: the relatively stereotyped form ("COPD") with a rapid fall in pulmonary function (especially FEV1), usually in heavy smokers with no apparent antecedent immunological factors non bronchial reactivity and the more variable form ("chronic asthmatic bronchitis") in those with "asthmatic" predisposing immunological characteristics. (The predictive valuate of the factors and their effects in young adults, prior to frank clinical AOD, and in middle bronchial reactivity will be evaluated, including familial factors, smoking, effects of environmental/occupational exposures and host immunological status. Likewise, the role as the high IgE in smokers, will be explore further; it is though that T-cell dysfunction related to smoking induces high IgE, which is associated with the declining pulmonary function, especially in the presence of bronchitic symptoms.